The invention relates to frequency converters, and in particular, to improvement of the waveform of the frequency converters' input supply current.
The input supply current of a typical 1- or 3-phase supplied diode rectifier is highly distorted. FIG. 1 shows an example of a frequency converter to be connected to a 1-phase supply network, the frequency converter consisting of a diode rectifier 1, a DC intermediate circuit 2 and an inverter capable of supplying 3-phase load.
One or more capacitors of the intermediate circuit on the DC voltage side of the rectifier constitute an energy storage of the inverter. At the same time the voltage charged in the capacitor limits the conduction time of a diode bridge, because, as known, energy is transferred to a DC voltage intermediate circuit only when a main voltage of the supplying network is higher than the DC voltage of the intermediate circuit. Consequently, energy is transferred to the intermediate circuit only at short periods close to peak value time instants of network voltage. Thus, the energy transferred to the intermediate circuit is transferred in high and short current spikes.
High current spikes increase the effective value of an input current of a device, whereby fuse protection and thermal rating of cables, for instance, must be oversized in relation to the actual efficiency to be utilized. In addition, high current spikes increase stresses of all components exposed thereto. FIG. 2 shows an example of a situation, where the 3.3 kW DC power of a rectifier having a capacitive DC voltage circuit energy storage is generated without additional filtering, whereby current spikes are only limited by impedances of a transformer and cables of the AC network. The maximum value of a current spike is about 100 A and the effective value is about 28.8 A. If the same average power were transferred in a sinusoidal current having the same phase with the voltage, the peak value of the current would only be about 20.3 A and the effective value about 14.3 A.
In order to reduce the current spikes, i.e. to filter the supply current, a passive choke is often added to the rectifier. A drawback with this procedure is that the size, weight and price of the choke required increase considerably as the need for filtering increases. Moreover, voltage loss taking place in the choke
  u  =      L    ⁢                  ⅆ        i                    ⅆ        t            increases along with the filtering need, because filtering capacity is added by increasing the choke inductance L.
One way to reduce current distortion is to replace the diode rectifier by a rectifier including active power factor correction, whereby the device always has supply current of good quality. A drawback with the active power factor correction is that the structure of the actual device, such as a frequency converter will change, and consequently, the manufacturing costs thereof will also often increase. The increase in costs poses a particular problem in a situation, where the quality of supply current is not relevant to the client, and the increased costs do not provide any actual added value for the client. A corresponding situation arises in connection with the quality standards of supply current, because the same product may be used in areas where highly different standards prevail, the standards being stricter in some areas regarding network effects.